1. Field of the Invention
The invention relates to devices that measure the electrical impedance of body tissues, and in particular an impedance pneumograph utilizing a cable and electrodes also used for ECG monitoring.
2. Description of the Prior Art
The electrical impedance of the thorax varies cyclically with each breath. This property has been exploited to monitor the respiration activity of a person, by monitoring the thoracic impedance. By detecting these variations in the thoracic impedance an instrument is able to monitor respiratory activity. The impedance of the thorax may be measured by means of electrodes placed on the chest. Single-function instruments that measure respiratory activity by means of impedance find wide commercial use, particularly in the areas of sleep apnea monitoring. However, respiration monitoring capability is frequently included as part of more complex monitoring devices which serve other functions as well. Since the monitoring of the electrocardiogram (ECG) also requires placement of electrodes on the chest, in positions useful for respiration monitoring, it has become common practice to integrate a respiration monitoring feature into ECG monitoring devices.
In cases where respiration monitoring and ECG are combined, the respiration monitor shares the cable, leadwires, and electrodes with the ECG function. The respiration and ECG monitoring subsystems must be designed such that they do not produce mutual interference. This places constraints on the design of the respiration circuit, and how it may be coupled to the patient through the ECG cable and electrodes. Many instruments accept ECG cables conforming to legacy designs, which predate widespread use of respiration monitoring. As such, the design of these cables is not optimized for respiration monitoring. This forces certain compromises in the design of the respiration monitoring subsystem. Further, there are several different cable constructions in common use, having varying impacts on the design and performance of the respiration circuit.
Due to the above practical limitations of the known art, most equipment manufacturers restrict the application of their respiration monitor to a particular class of cables for which their design has been optimized. In cases where a manufacturer wishes to accept widely differing cables, such as to accept the cables used by a different manufacturer, it may be necessary to modify or recalibrate the respiration circuit for optimal performance. Even when this is not the case, the present art restricts the user to a limited set of cables and accessories, which were contemplated when the respiration circuit was designed and calibrated. Greater flexibility in the application of a respiration monitoring subsystem would be possible if the device were capable of accepting a broad range of cable parameters and patient conditions without the need for modification or recalibration.
Accordingly, it is an object of the invention to produce an impedance respiration monitoring circuit, the sensitivity of which does not vary appreciably over a wide range of cable shunt capacitance and series resistance, and patient baseline resistance.
It is another object of the invention to produce an impedance respiration monitoring circuit, which can be coupled to existing ECG electrodes with minimal degradation of ECG performance.
To achieve the foregoing and other objects, the invention, in accordance with certain of its aspects, provides an improved method for measuring the resistive component of the combined total impedance of the patient, electrodes, and cable, said method comprising the steps of:
(a) injecting as an impulse a known quantity of charge into the total impedance to be measured, causing a voltage to be developed across said impedance; and
(b) integrating said voltage, the period of integration being substantially longer than the exponential decay time constants of said impedance.
In accordance with other of its aspects, the invention provides apparatus for practicing the foregoing method, which comprises:
(a) a voltage generator delivering a substantially rectangular pulse or square wave, having a half-period significantly longer than the exponential decay time constants of the impedance to be measured;
(b) an essentially capacitive coupling network differentiating said pulse or square wave, and injecting charge to the impedance to be measured;
(c) a receiving circuit, consisting of either an asynchronous rectifier or synchronous demodulator, which detects the voltage developed across the impedance to be measured following the injection of the charge; and
(d) a low pass filter having a time constant much longer than the period of generator (a).
To the accomplishment of the above and related objects the invention may be embodied in the form illustrated in the accompanying drawings. Attention is called to the fact, however, that the drawings are illustrative only. Variations are contemplated as being part of the invention, limited only by the scope of the claims.